FIG. 1 is a block diagram showing the configuration of the conventional analog and digital television (TV) receiving sets. The signal input from the antenna 61 is distributed either to system A for receiving an analog broadcasting or to system D for receiving the digital broadcasting.
When analog broadcasting with an FM modulation system, the channel is chosen by selecting the signal input from antenna 61 through channel selector 62. The signal thus selected is input into FM demodulator 68 to be output as FM-demodulated signal 6a. The demodulated signal 6a is received by video processor 64 to be output as the video signal 6d.
When digital broadcasting using a multiple PSK modulating system, the broadcasting channel is chosen by selecting the signal input from antenna 61 through channel selector 65. The signal thus selected is input into I/Q wave detector 66 to be output as an I/Q-detected output signal 6c. The I/Q-detected signal 6c is received by digital demodulator 67 to be output as the analog video signal 6b, and as a video output signal.
Thus, the analog and digital broadcasts can be received by different television receivers. The users however could not determine whether the received telecast was based on an analog or on digital signal until the video signal was transformed into the actual picture on the screen. Further it was necessary for users to change the telecasting system to match the signal received even when they could identify the system.
In the foregoing conventional broadcasting system discriminating television receiver which distinguishes an analog telecast from digital one, it was impossible to identify the telecast system before the image came out on the screen as described above. Also users were compelled to change the system to match the signal received once they were able to determine the type of system, analog or digital. Such a switching operation was inconvenient and time-consuming.
FIG. 2 represents another conventional television receiver system that operates with both analog and digital systems and which is conceived from the conventionally known, different television receivers responding to analog and digital modulations.
In FIG. 2, the television signal input into the input terminal 1001 is selected by channel selector 1002 as the IF (Intermediate Frequency) signal for either digital demodulation for digital signals received or analog demodulation for analog signals received. When a digital signal is received, the wave detector 1003 (composed of the detectors 1003a and 1003b and 90.degree. phase shifter 1003c) detects the IF signal using the regenerative carrier signal of the regenerative carrier oscillator 1004. The output thus detected is output to the digital demodulation portion in the following stage respectively through the output terminals 1005a and 1005b.
When an analog signal is received, the IF signal is fed to wave detector 1006, where it is detected by the carrier signal of regenerative carrier oscillator 1007, and then output at output terminal 1008. The wave detect output of terminal 1008 is a direct current voltage corresponding to the frequency of IF signal for analog and to that of the regular IF signal. A signal corresponding to the channel select deviation of the channel selector 1002 can be obtained by supplying the output of terminal 1008 to low-pass filter 1010 through dc amplifier 1009. This output, fed back as an AFC output to channel selector 1002, contributes to enhanced accuracy of channel selection.
Such a conventional system is critically vulnerable when incorporated into an IC system. Regenerative carrier oscillator 1004 of the digital portion requires an extremely accurate phase characteristic to enable use of a phase modulation system such as multi-valued PSK. It is very difficult to design an oscillator to output a regenerative carrier signal having a high frequency signal output with good phase characteristic. It is therefore not very feasible to incorporate the above system into an integrated circuit system. Forming the system with discrete units rather than an IC design, results in increases in size and manufacturing cost.
On the other hand, on AFC which acquires data on channel selection deviation at the analog portion requires regenerative carrier oscillator 1007 to generate an extremely precise oscillation frequency. The required precision renders circuit-integration difficult since some exterior adjustment must be made for dispersion in device manufacturing, reducing the yield due to the frequency drift in the oscillator. These factors render IC incorporation virtually impossible.
In another example of a conventional broadcast discriminating television receiver, where phase modulation, if any, is used for a digital modulation system, a regenerative carrier oscillator must generate a high frequency output signal having an extremely accurate phase characteristic. This factor rendered circuit-integration of the system more difficult. Moreover, the conventional AFC using the analog portion to determine the channel select deviation required a regenerative carrier oscillator that generated and extremely accurate oscillation frequency as well as some external regulation against a dispersion in the manufacturing of devices.